1. Field of the Invention
The present invention relates to a permanent magnet rotating electric machine, and more particularly to a motor for use in an electric power steering device for automobile, for example.
2. Description of the Related Art
In the related-art permanent magnet rotating electric machine, a torque pulsation called a cogging torque occurs due to an interaction between a stator core and a permanent magnet. Since the cogging torque causes vibration, it is strongly desired to decrease the cogging torque. Therefore, various techniques for decreasing the cogging torque have been proposed, and the configuration of permanent magnet to decrease the cogging torque has been examined. For example, as one example of a motor having twelve permanent magnets and nine magnetic poles, the curved surface of permanent magnet opposed to a stator is like a circular arc, the value of the radius of a rotor divided by the curvature of the surface of permanent magnet opposed to the stator is set to the value for decreasing the cogging torque (e.g., refer to JP-A-2005-341688, page 3-4, FIGS. 3 and 4). Also, a permanent magnet motor in which the number of poles is 6 and the number of teeth is 18 has the center of outer diameter eccentric so that the outer diameter of permanent magnet passing the outside contour may be smaller than the outer diameter of adjacent permanent magnet passing the vertex of contour, and the surface shape of the rotor is like a petal (e.g., refer to JP-A-2000-350393, page 3, FIG. 1).
In the related-art permanent magnet rotating electric machine, though the cogging torque can be decreased by making the curved surface of permanent magnet opposed to the stator like a circular arc, there is a problem that the height of the end portion of permanent magnet in the peripheral direction is smaller, whereby the irreversible demagnetization at the end portion of permanent magnet in the peripheral direction is likely to occur. When the permanent magnet rotating electric machine operates at the high temperature, there is another problem that the irreversible demagnetization is more likely to occur because the coercive force of permanent magnet is decreased. If the irreversible demagnetization occurs, the torque is decreased, and the cogging torque and the torque ripple are increased because the magnetic flux produced by the permanent magnet is different from at the time of design, causing the vibration or noise. Therefore, if it is used for the electric power steering device for automobile, the good steering feeling can not be obtained.